The present invention relates to food containers and methods and apparatus for making food containers, and more particularly relates to tubular containers wound from at least one paperboard body ply and a liner ply.
Food and drink products and other perishable items are often packaged in tubular containers which are sealed at both ends. These tubular containers typically include at least one structural body ply and are formed by wrapping a continuous strip of body ply material around a mandrel of a desired shape to create a tubular structure. The body ply strip may be spirally wound around the mandrel or passed through a series of forming elements so as to be wrapped in a convolute shape around the mandrel. At the downstream end of the mandrel, the tube is cut into discrete lengths and is then fitted with end caps to form the container.
Tubular containers of this type typically include a liner ply on the inner surface of the paperboard body ply. The liner ply prevents liquids such as juice from leaking out of the container and also prevents liquids from entering the container and possibly contaminating the food product contained therein. Preferably, the liner ply is also resistant to the passage of gasses, so as to prevent odors of the food product in the container from escaping and to prevent atmospheric air from entering the container and spoiling the food product. Thus, the liner ply provides barrier properties and the body ply provides structural properties.
Conventional liner plies most often include aluminum foil which has good barrier properties and also has advantageous strength properties. In particular, the liner is wound onto the mandrel prior to the winding of the body ply and must be sufficiently strong and stiff to be independently wound on the mandrel without stretching or wrinkling. Because of the support provided by the foil layer of the liner, such liners are known as xe2x80x9csupportedxe2x80x9d liners.
One or more polymeric layers are normally adhered to the foil to further improve the barrier properties of the liner and it is sometimes the case that the foil layer is not necessary for barrier properties but is included in the liner only to provide support. Such foils are expensive and thus it is desired to provide an xe2x80x9cunsupportedxe2x80x9d liner having the requisite barrier properties without the aluminum foil layer. However, because of the problems associated with winding an unsupported liner on the mandrel, such as stretching, creasing or other misshaping of the liner, it has not been commercially feasible with conventional winding apparatus and methods to manufacture a container having an unsupported liner ply.
Another problem associated with conventional liners is excessive friction between the liner ply and the mandrel as the tubular container is wound. If the amount of friction is too high, the mandrel can suffer from overheating or premature wear. Lubricants are used although the lubricants can be expensive and may present FDA concerns. Accordingly, it would be desirable to provide a tubular container which does not create excessive friction with the shaping mandrel.
In addition, the aluminum foil layer typically includes a kraft paper backing for allowing the foil layer to be adhered to the paperboard body ply. Aqueous based adhesives (or xe2x80x9cwet adhesivesxe2x80x9d) are preferably being used to adhere the liner ply to the body ply because solvent based adhesives have become disadvantageous in light of various environmental concerns. However, it has heretofore been difficult to get the aqueous adhesives to stick to the smooth and impervious surface of the aluminum foil layer. Accordingly, a kraft paper backing has been preadhered to the foil layer so that the liner can be adhered to the paperboard body ply with wet adhesives. However, the kraft paper adds further cost and thickness to the liner.
The liner ply is sealed to itself along a helical seam which is typically slightly offset from the helical seam of the body ply. Wet adhesives have typically not been able to adhere directly to the foil layer as discussed above, and thus the liner ply seam is formed with an xe2x80x9canacondaxe2x80x9d fold, wherein the overlying edge of the liner ply is folded back on itself and adhered to the underlying edge. The anaconda fold allows the polymeric layers on the surface of the foil layer to be heat sealed together. Alternatively, a hot melt adhesive can be used to seal the anaconda fold of the overlying edge of the liner ply to the underlying edge. An additional advantage of the anaconda fold is that the edge of the kraft paper is not exposed to the interior of the container and thus liquids in the container will not be absorbed by the kraft paper. An example of such a fold is illustrated in U.S. Pat. No. 5,084,284 to McDilda, et al.
Anaconda folds are undesirable, however, because of their increased thickness. The thickness of an anaconda fold seam is equal to three thicknesses of the liner ply and poses difficulties when attempting to hermetically seal the ends of the tubular container. Specifically, the ends of the tube are often rolled outwardly after being cut so as to form a rolled circular bead or flange on one or both ends of the tube and then end caps or covers are sealed to the bead with an adhesive sealant or compound. However, in the area where the thick anaconda fold seam forms a portion of the edge surface, the end surface of the bead or flange can be substantially non-planar thus forming hill-like and/or valley-like irregularities. Accordingly, an extra amount of adhesive sealant must be applied to the edge surface at least in the area of the anaconda fold seam to fill the discontinuities and hermetically seal the tubular container. The additional application of adhesive sealant is disadvantageous because of the extra sealant which must be used and the increased difficulty in removing the seal by the consumer due to the additional adhesive sealant.
Prior tubular containers having a liner without an anaconda fold seam include the container disclosed in U.S. Pat. No. 3,520,463 to Ahlemeyer. The container disclosed therein includes a liner ply of aluminum foil which is coated on one surface to inhibit chemical attack. The liner ply web is fed to a pair of combining rolls where its uncoated surface is forced into contact with an adhesively coated surface of a body ply web. Adhesives are disclosed and include animal glue, casein-latex emulsion, vinyl-copolymer emulsion, and sodium silicate. The composite web is then spirally wound into tubular form about a mandrel to create a continuous tube. The overlapping edges of the liner ply are secured together with a hot melt adhesive.
Accordingly, it would be desirable to provide methods and apparatus for manufacturing a tubular container having an unsupported liner ply which does not include a foil layer. In addition, it would be highly desirable to provide such a container wherein the liner ply is securely adhered to the body ply with an aqueous adhesive. Such an aqueous adhesive would avoid the problems associated with solvent adhesives but should be capable of forming a sufficiently strong bond with the liner ply, a construction which has not yet been commercially feasible. In addition, it would be advantageous if the liner ply could be sealed without using an anaconda fold seam. A tubular container which could be wound without generating excessive friction with the shaping mandrel would also be very desirable.
These and other objects and advantages are met by the present invention which include methods and apparatus for manufacturing a tubular container having a paperboard body ply and a polymeric liner ply adhered thereto with a wet adhesive. In particular, the advantageous method according to the present invention includes the steps of applying an aqueous adhesive to a paperboard body ply and then heating the aqueous adhesive to evaporate at least part of the water content to render the adhesive substantially tacky. After the heating step, the paperboard body ply and a polymeric liner ply are passed through a pair of nip rollers to adhere the liner ply to the body ply. The body ply and the adhered liner ply are then wrapped around the shaping mandrel to create the tubular container. The body ply may be wrapped helically around the mandrel to create a spirally wound tube or wrapped longitudinally around the mandrel to create a convolute tube.
Preferably, the heating step includes subjecting the adhesive to at least about 100,000 J/m2 and more preferably at least about 460,000 J/m2. A sufficient amount of heat can be imparted by advancing the body ply adjacent to a heat source having the capability of generating a heat flux of at least about 50,000 W/m2 for a period of less than about 3 seconds. This amount of heat will raise the temperature of the paperboard above the boiling point of water to evaporate part of the water content from the aqueous adhesive.
Another advantageous method includes advancing a continuous polymeric liner ply having first and second marginal edge portions and an adhesive layer on the first marginal edge portion of the surface of the liner ply facing the body ply. The adhesive layer includes a non-aqueous polymeric adhesive which is activated at a predetermined activation temperature. The liner ply and body ply are then passed through a pair of nip rollers and are aligned such that the first marginal edge portion of the liner ply extends beyond a first edge of the body ply.
The body ply and adhered liner ply are wrapped around the shaping mandrel and the non-aqueous adhesive layer of the liner ply is heated to a temperature above the activation temperature of the adhesive. Heating of the non-aqueous adhesive layer occurs prior to the body ply and liner ply being wrapped around the shaping mandrel, or while the plies are wrapped on the shaping mandrel, or both. The body ply is further wrapped around the shaping mandrel so that the second marginal edge portion of the liner ply is adhered to the first marginal edge portion by the non-aqueous adhesive layer, thus creating a sealed liner. Advantageously, the mandrel may also be heated.
An apparatus for manufacturing multi-ply tubular containers for food products having a paperboard body ply and an inner polymeric liner ply also forms a part of the invention. The apparatus includes a supply of continuous body ply material and an adhesive applicator adjacent to one surface of the body ply for applying an aqueous adhesive to that surface. The apparatus further includes at least one heat source downstream of the adhesive applicator for heating the aqueous adhesive and which is capable of evaporating enough water to render the adhesive substantially tacky. Preferably, the heat source comprises an infrared heater capable of generating heat flux of at least about 50,000 W/m2.
The apparatus also includes a supply of continuous liner ply material and a pair of nip rollers through which the liner ply and the body ply are passed to adhere the liner ply to the body ply. In a preferred embodiment, a corona discharge element is positioned adjacent to the surface of the liner ply to promote adhesion to the body ply. The body and liner plies are wrapped around a shaping mandrel downstream of the nip rollers to form the body and liner plies into the desired tubular shape. A cutting station adjacent to the mandrel cuts the thus formed wound tube into discrete container lengths.
According to another aspect of the invention, the liner ply has a substantially smooth surface prior to the body ply being wrapped but has a wrinkled surface after being wrapped which is caused by circumferential compression of the liner ply. The peaks and valleys of the wrinkled surface cause the liner ply to move easily over the shaping mandrel, thus advantageously decreasing friction between the liner ply and the mandrel and eliminating problems of premature wear and overheating associated with conventional containers.
The body ply defines a predetermined circumferential length before being wrapped corresponding to one revolution of the body ply around the shaping mandrel when wrapped in the tubular shape. The liner ply is adhered to the inner surface of the body ply and defines a circumferential length which is equal to that of the body ply prior to being wrapped. Accordingly, the liner ply is circumferentially compressed when the body ply is wrapped into the tubular shape and the wrinkled surface is created.
The barrier layer preferably includes a layer of polyester having at least one metallized surface. In addition, the liner ply preferably has a thickness of less than about 3 mils. According to another embodiment, a second paperboard ply may be adhered to the first body ply opposite the liner ply for added strength.